Light barriers comprising a light source and a light receiver, e.g. for the detection of passing objects, are generally known. Further, it is known that gas bubbles contained in a liquid column are detectable since their optical density is different from that of the liquid column. Therefore, gas bubbles in a liquid column can be detected by means of a suitable light barrier. An apparatus for this purpose is described by WO-A-97/33154.
With respect to automatic analyzers for the analysis of liquids, it is known that these liquids are handled in small quantities, e.g. in portions of minimally 1 to 2 microliters, by aspiration into and ejection out of pipetting needles. In the analysis process, the appearance of bubbles in the needles constitutes an important source of errors since such bubbles can strongly influence the quantities of the ejected liquids.